This invention relates to a two-stage centrifugal compressor in which compressor impellers are mounted directly on a motor shaft.
Japanese Patent Examined Publication No. 5-36640 discloses a two-stage centrifugal compressor, used in a plant or a factory, in which in order to achieve a compact design of the two-stage centrifugal compressor, impellers are mounted respectively on opposite ends of a shaft of an electric motor, and are driven directly without the use of any speed increaser. In the centrifugal compressor disclosed in this publication, a compressor casing, having a discharge scroll, is separate from a motor casing having a motor stator and bearings mounted therein. A downstream side of the discharge scroll is connected to a pipe through a flange. Each of first and second compressor stages has an suction portion for drawing gas in an axial direction, and is connected to an suction pipe through a flange.
In order to achieve a compact design of a centrifugal compressor, there has also been proposed the type of centrifugal compressor in which a compressor casing, an intermediate cooler and a discharge cooler are formed integrally with one another. One such example is disclosed in Japanese Patent Unexamined Publication No. 7-103162. The compressor, disclosed in this publication, comprises two parallel rotation shafts (that is, a low-speed shaft and a high-speed shaft) disposed at the same height, and the two shafts are connected together through a speed-increasing gear (speed increaser). A housing of the speed-increasing gear, is vertically divided into two portions (that is, upper and lower portions) at a horizontal plane including centerlines (axes) of the two shafts. The lower housing is formed integrally with a cooler and so on, and the upper housing is removable. Flanges for mounting a motor thereon are formed respectively on one ends of the upper and lower housings, and the coaxial relation between the low-speed shaft and the motor is secured through this flange.
In the centrifugal compressor, disclosed in Japanese Patent Examined Publication No. 5-36640, the compact design of the apparatus can be achieved. However, the compressor casing and the motor casing are separate from each other, and the number of the component parts, including those interconnecting the two casings, is increased, and therefore machining or working errors and assembling errors for the parts are accumulated during the assembling operation, which leads to a possibility that in a bearing-holding portion inside a stator, the coaxial relation between a rotor and the stator, as well as the precision of axial positioning of the rotor and the stator, is lowered. In the case where magnetic bearings and impellers without a shroud are used in order to achieve the more compact design of the centrifugal compressor disclosed in this publication, it is difficult to keep the positional relation between the rotor and the stator within a predetermined allowable range, although it is necessary to accurately position the rotor and the stator with respect to each other. Therefore, in order to enhance the assembling precision, if the parts are more accurately positioned by fitting portions, or if the parts are fixedly positioned by knock pins or the like, the assembling time and the machining time are increased, and the advantageous effects, such as the reduction of the cost by the compact design and the enhanced reliability, are adversely affected.
And besides, the centrifugal compressor, disclosed in this publication, is connected to the pipe through the flange at the downstream side of the discharge scroll of each compressor stage, and not only the first-stage compressor but also the second-stage compressor draw the gas in the axial direction, and are connected to the pipe through the flange, and the arrangement of pipes between the compressor and a cooler is inevitably complicated. Therefore, despite the fact that the impellers are mounted directly on the motor shaft, the advantageous effect, obtained with the space-saving design of the package compressor, including the cooler, can not be fully achieved. Incidentally, the compressor body and the cooler are different from each other in the amount of thermal deformation during the operation, and therefore there are possibilities that the air-tightness is lowered by the thermal deformation, and that a leakage results therefrom.
The centrifugal compressor, disclosed in Japanese Patent Unexamined Publication No. 7-103162, is the two-shaft compressor having the speed-increasing gear, and no consideration is given to a construction in which the motor is disposed coaxially with the rotation shaft of the compressor, and the impellers are driven directly without the use of any speed-increasing gear in order to achieve the compact design of the centrifugal compressor.